I 12 2 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



-4 -2 ~0~ +2 +4 



RIGHT ATRIAL PRESSURE ImmHg) 



jj! +50 +40 +30 +20 +10 -10 



LEFT ATRIAL PRESSURE (mm Hg) 



fig. 27. Effect of reduced blood volume on cardiovascular 

 dynamics. This figure shows that the mean systemic pressure 

 and mean pulmonary pressure are both greatly reduced, thus 

 causing corresponding decreases in the two venous return 

 curves. 



mean pulmonary pressure to 2.5 mm Hg. Thus, the 

 net effect of a decrease in blood volume is simply to 

 reduce both the mean systemic and mean pulmonary 

 pressures. This does not affect, at least temporarily, 

 the output curves of either the left or the right ven- 

 tricles until sympathetic reflexes occur. Also, it does 

 not affect, at least temporarily, the slopes of the two 

 venous return curves. Therefore, the only significant 

 effect is a reduction in both the systemic and pul- 

 monary venous return curves because of the reduced 

 mean systemic and mean pulmonary pressures. As a 

 consequence, both the right ventricular and left 

 ventricular outputs are reduced, in this instance to 

 approximately 55 per cent of normal. 



summary of the complex analysis. This more com- 

 plex analysis of the circulation has been presented 

 to illustrate a method for analyzing the effects of 

 unilateral excess load or unilateral alteration in pump- 

 inn effectiveness of the heart. It has particular im- 

 portance in analyzing abnormalities of the pulmonary 

 circulation. On the other hand, as one can readily 

 see from the last few figures, even when relativelv 

 large quantities of blood shift into or out of the pul- 

 monary circulation, rather small changes occur in 

 the dynamics of the svstemic circulation. Therefore, 



from a practical point of view, when one is concerned 

 principally with systemic effects of the circulation, 

 the simplified analysis is usually quite adequate. 



Obviously, only a few examples of the vast number 

 of uses of these two types of analysis have been given. 

 Because of the multitude of different quantitative 

 values that can be assumed by different venous re- 

 turn and different output curves, the analyses can 

 likewise assume literally thousands of different forms. 

 However, the various alterations in the individual 

 curves that can occur under manv different circu- 

 latory conditions obey rather simple principles. 

 Therefore, in almost any circulatory condition, one 

 can either establish the different curves experimentally 

 or can predict them very accurately, and from these 

 he can proceed with an analysis of the different effects 

 which will occur in the circulation, particularly as 

 they relate to venous return, cardiac output, left and 

 right atrial pressures, pulmonary blood volume, and 

 svstemic blood volume. 



SPECIFIC FACTORS THAT AFFECT VENOUS RETURN 



Thus far, we have considered only a general analy- 

 sis of venous return. Now we need to consider several 

 factors that at times play highly significant and spe- 

 cific roles in the local process of blood flow along 

 the veins. These include especially the effects of a) 

 the venous pump, b) the collapse factor, c) central 

 pressure pulsations, and d) local factors in the tissues 

 that help to govern venous return such as local tissue 

 activity and tissue utilization of oxygen. 



EFFECT OF THE VENOUS PUMP ON VENOUS RETURN. Al- 

 most every student of physiologv is already familiar 

 with the function of the so-called "venous pump." 

 That is, all peripheral veins beyond the visceral cavi- 

 ties are supplied with valves oriented toward the 

 heart, and any factor that causes successive compres- 

 sions of the veins exerts a pumping action that propels 

 blood toward the heart. The different types of com- 

 pression that have been implicated in the venous 

 pump include a) compression incident to muscular 

 movement either as a result of direct muscular pres- 

 sure on the veins or indirectly as a result of movements 

 of the joints and tissues, and b) pulsatile compression 

 of the veins caused by arteries lying in the same 

 sheaths as the veins. The second of these has not 

 proved to be of any particular significance. Therefore, 

 the venous pump is also frequently called simply the 

 "muscle pump" (16, 17, 27, 49, 155, 156, 191). 



